Dante wants to ask how shades can grow thin on the sixth terrace — if they have no physical bodies, they have no flesh to waste. Virgil grants him permission and turns the question to Statius, who delivers the longest sustained philosophical passage in Purgatorio: a theory of embryology and soul-formation drawn from Aristotle, Avicenna, and Christian theology. The key point is this: when the soul departs the body at death, it impresses itself upon the surrounding air the way heat shapes the air around it, forming what Dante calls the "shadow body" — an aerial form that can feel, suffer, laugh, and weep even without flesh. This is how Dante and Statius can see shades growing thin: the aerial body mirrors what the soul would experience in flesh.
The seventh terrace: a wall of fire that stretches across the entire path. There is no going around it. Through the fire, angels can be heard singing. Voices cry out examples of chastity: "I know not man" — Mary's words to Gabriel. Then "Diana kept the woods pure" — the chaste huntress goddess driving away any who violated her grove's purity. And the names of wives and husbands who were faithful within marriage. The fire burns; the voices call; and Dante and his companions have not yet entered.